What is Mendel’s Law of Inheritance? (NCERT Notes)

Mendel’s Law of Inheritance : Introduction

Mendel’s Laws of Inheritance are the basic principles that explain how traits are passed from parents to offspring. These laws were proposed by the Austrian scientist Gregor Mendel after conducting experiments on pea plants.

Because of his pioneering work in genetics, Gregor Mendel is known as the Father of Genetics.

Who Was Gregor Mendel?

Key Facts

Gregor Mendel was an Austrian monk and scientist.
He performed experiments on pea plants (Pisum sativum).
His experiments were conducted between 1856 and 1863.
He studied the inheritance of visible traits.
His observations led to the formulation of inheritance laws.

Why Did Mendel Choose Pea Plants?

Characteristic	Reason
Easy to grow	Requires little maintenance
Short life cycle	Produces generations quickly
Self-pollination	Maintains pure lines
Cross-pollination possible	Easy to perform controlled breeding
Distinct traits	Easy observation of inheritance

Traits Studied by Mendel

Mendel selected seven pairs of contrasting traits.

Trait	Dominant Character	Recessive Character
Seed Shape	Round	Wrinkled
Seed Color	Yellow	Green
Flower Color	Violet	White
Pod Shape	Inflated	Constricted
Pod Color	Green	Yellow
Flower Position	Axial	Terminal
Plant Height	Tall	Dwarf

Important Terms

Gene

A unit of heredity.
Controls a specific trait.

Alleles

Different forms of the same gene.
Example: Tall (T) and Dwarf (t).

Dominant Trait

Trait expressed in the presence of another trait.
Example: Tallness (T).

Recessive Trait

Trait expressed only when both alleles are recessive.
Example: Dwarfness (t).

Genotype

Genetic makeup of an organism.
Example: TT, Tt, tt.

Phenotype

Physical appearance of an organism.
Example: Tall or Dwarf.

Mendel’s Laws of Inheritance

Mendel proposed three major laws:

Law of Dominance
Law of Segregation
Law of Independent Assortment

1. Law of Dominance

Statement

When two contrasting traits are present together in an organism, only one trait is expressed while the other remains hidden.

Example

Cross:

Tall Plant (TT) × Dwarf Plant (tt)

F₁ Generation

All offspring become:

Tt (Tall)

Observation

Tallness appears.
Dwarfness remains hidden.

Conclusion

Tall trait = Dominant
Dwarf trait = Recessive

2. Law of Segregation

Statement

The two alleles of a gene separate during gamete formation, so each gamete receives only one allele.

Example

Parent genotype:

Gametes formed:

During fertilization, these alleles combine randomly.

Significance

Recessive traits can reappear in later generations.
Explains inheritance patterns accurately.

Monohybrid Cross

A cross involving only one pair of contrasting traits is called a Monohybrid Cross.

Example

Tall (TT) × Dwarf (tt)

F₁ Generation

All Tall (Tt)

F₂ Generation

Genotype	Number
TT	1
Tt	2
tt	1

Genotypic Ratio

1 : 2 : 1

Phenotypic Ratio

3 Tall : 1 Dwarf

3. Law of Independent Assortment

Statement

Different pairs of traits are inherited independently of one another.

Example

Traits:

Seed Shape (Round/Wrinkled)
Seed Color (Yellow/Green)

Inheritance of seed shape does not affect inheritance of seed color.

Importance

Creates genetic variation.
Produces diverse combinations of traits.

Dihybrid Cross

A cross involving two pairs of contrasting traits is called a Dihybrid Cross.

Example

Round Yellow (RRYY) × Wrinkled Green (rryy)

F₂ Phenotypic Ratio

Phenotype	Ratio
Round Yellow	9
Round Green	3
Wrinkled Yellow	3
Wrinkled Green	1

Phenotypic Ratio

9 : 3 : 3 : 1

Significance of Mendel’s Laws

Importance in Genetics

Foundation of modern genetics.
Explains inheritance of traits.
Helps understand genetic disorders.
Useful in plant breeding.
Useful in animal breeding.
Helps predict offspring characteristics.

Limitations of Mendel’s Laws

Not all traits follow simple dominance.
Some traits show incomplete dominance.
Some traits show codominance.
Many traits are controlled by multiple genes.
Environmental factors can influence traits.

Quick Summary Table

Law	Main Idea
Law of Dominance	Dominant trait masks recessive trait
Law of Segregation	Alleles separate during gamete formation
Law of Independent Assortment	Different traits inherit independently

Practice Questions

Very Short Answer Questions

1. Who is called the Father of Genetics?

Answer: Gregor Mendel.

2. Which plant did Mendel use for his experiments?

Answer: Pea plant (Pisum sativum).

3. What is a gene?

Answer: A unit of heredity that controls a trait.

4. What is a dominant trait?

Answer: A trait that is expressed even in the presence of another contrasting trait.

5. What is a monohybrid cross?

Answer: A cross involving one pair of contrasting traits.

Short Answer Questions

1. State the Law of Dominance.

Answer: When two contrasting traits are present together, only the dominant trait is expressed while the recessive trait remains hidden.

2. Why did Mendel choose pea plants?

Answer: Pea plants are easy to grow, have a short life cycle, possess distinct traits, and allow both self-pollination and cross-pollination.

3. What is the phenotypic ratio of a monohybrid cross?

Answer: 3 : 1.

4. What is the genotypic ratio of a monohybrid cross?

Answer: 1 : 2 : 1.

5. State the Law of Segregation.

Answer: The two alleles of a gene separate during gamete formation so that each gamete receives only one allele.

Fill in the Blanks

Gregor Mendel is known as the Father of Genetics.
Mendel performed experiments on pea plants.
Tallness in pea plants is a dominant trait.
Dwarfness is a recessive trait.
The phenotypic ratio in a monohybrid cross is 3:1.
The genotypic ratio in a monohybrid cross is 1:2:1.
A cross involving one trait is called a monohybrid cross.
A cross involving two traits is called a dihybrid cross.
Alleles separate according to the Law of Segregation.
Independent inheritance of traits is explained by the Law of Independent Assortment.

MCQs (Multiple Choice Questions)

1. Who proposed the laws of inheritance?

A. Darwin
B. Mendel
C. Lamarck
D. Watson

Answer: B. Mendel

2. Which plant was used by Mendel?

A. Wheat
B. Rice
C. Pea
D. Maize

Answer: C. Pea

3. Which trait is dominant in pea plants?

A. Dwarf
B. Tall
C. Green seed
D. White flower

Answer: B. Tall

4. The Law of Dominance explains:

A. Mutation
B. Evolution
C. Expression of dominant traits
D. DNA replication

Answer: C. Expression of dominant traits

5. The phenotypic ratio of a monohybrid cross is:

A. 1:2:1
B. 3:1
C. 9:3:3:1
D. 2:1

Answer: B. 3:1

6. The genotypic ratio of a monohybrid cross is:

A. 3:1
B. 2:1
C. 1:2:1
D. 9:3:3:1

Answer: C. 1:2:1

7. Which law explains separation of alleles?

A. Dominance
B. Segregation
C. Evolution
D. Mutation

Answer: B. Segregation

8. A dihybrid cross involves:

A. One trait
B. Three traits
C. Two traits
D. Four traits

Answer: C. Two traits

9. The F₂ phenotypic ratio in a dihybrid cross is:

A. 3:1
B. 1:2:1
C. 9:3:3:1
D. 1:1

Answer: C. 9:3:3:1

10. Mendel’s work laid the foundation of:

A. Ecology
B. Genetics
C. Botany
D. Taxonomy

Answer: B. Genetics

Key Takeaway

Mendel’s Laws of Inheritance explain how traits pass from parents to offspring. The Law of Dominance, Law of Segregation, and Law of Independent Assortment form the foundation of genetics and remain central concepts in modern biology and NCERT science education.